An apparatus of this type is described in U.S. Pat. No. 1,904,156 of Apr. 18, 1933. In this patent, the throughput-modulating member consists of an aerodynamically profiled toric core suspended by intercrossing metal wires stretched across the stream of driving fluid in which this profiled torus is immersed and which passes right through the electromagnetic command transducer.
A loudspeaker of this kind has a performance which is limited in fidelity and volume of sound. In effect, since there is no such thing as perfect aerodynamic profiling, noise perturbations are unavoidably produced by the particular surface of the modulator torus which is immersed in the flow, and by its suspension wires which pass through it; these perturbations are superimposed on the modulation representing the sound signal to be transmitted and impair the quality of the signal/noise ratio. Furthermore, the parallelism or coaxiality of the flow to be modulated and of the motion of its modulator member make it necessary to restrict the stream of driving fluid to a very thin lamina if even the smallest axial displacements of the modulator are to result in appreciable changes in stream contraction.
French Patent 78/33,178, published under U.S. Pat. No. 2,442,565, and filed on Nov. 24, 1978 enables these disadvantages to be overcome and offers a loudspeaker of the same type but with a performance which is greatly improved both as regards fidelity and volume of sound. According to this latter patent, the pneumatic circuit, on the one hand, and the electromagnetic transducer, on the other hand, are separated from each other over their whole length by a tranverse wall forming a partition at right angles to the axis of motion of the annular member in the annular throat, which forms an obstacle projecting above the said transverse wall and movable lengthwise across the said annular throat, while the pressurized fluid coming from the fluid inlet of the pneumatic circuit arrives at this annular throat tangentially relative to the transverse wall, that is to say at right angles to the said lengthwise-movable obstacle.
Flow control of a pressurized fluid by the use of a movable obstacle is well known in the field of high sound level sirens and noise generators, and reference may be made on this subject to French Patent 1,531,690, filed on May 25, 1967.
It should be noted, however, that the latter concerns a different technical field, which naturally does not involve the concepts of fidelity and signal/noise ratio, which are specific to loudspeakers, in the case of which:
on the one hand, the frequency operating range must cover the speech spectrum, that is to say the range 300-3,000 Hz;
on the other hand, the signal/noise ratio, which expresses the clarity or the intelligibility of the transmitted signal, must be at least 10 dB over the entire abovementioned operating range.
In French Patent 1,531,690, the movable obstacle has two positions: a projecting position which produces the displacement of an entry jet and a retracted position which produces no displacement. The effect produced by this obstacle is therefore to chop the continuous entry jet into two pulsating output flows, by alternate switching of the said entry jet towards either of two outlet conduits, finally producing a train of waves which are coarse--that is to say unmodulated--and consequently unsuitable for transmitting complex, and especially voice, messages.
In the loudspeaker forming the subject of French Patent 2,442,565, the modulation method employed has a low inertia while ensuring high modulation amplitudes, that is to say it makes it simultaneously possible to preserve the fidelity of vocal quality while ensuring a high retransmission level with a high signal/background noise ratio.
In this case, throughput modulation consists of throughput fluctuations about a mean throughput which is discharged via the same conduit which transmits the sound waves to the outside. This modulation is produced in a single stage in which the pneumatic circuit and the transducer are separated by a wall: on one side of the said wall the pneumatic circuit incorporating a throat extended by a diffuser nozzle, itself extended by an acoustic horn; on the other side of the wall, the transducer whose movable member controls an annular obstacle which projects from this wall, in the plane of the throat of the annular nozzle, the movable obstacle being joined to the said wall by a continuous elastic connection. Furthermore, this wall may incorporate orifices which permit the passage of a flow of fresh air induced by the flow of the driving fluid at the surface of the said wall (thus producing an ejector action) so that efficient cooling of the coil is ensured naturally, permitting the use of hot gases as a driving fluid.
In all these devices of the prior art, difficulties which cause problems may arise in use. Thus, in particular:
since the movable equipment consisting of the winding and its annular support are continuously driven with high-frequency oscillations, these alternating motions run the risk of eventually causing distortions of the support and uncoupling of the winding, causing breakdown of the apparatus;
the elastic connections for supplying the winding electrically have a characteristic frequency which can interfere with that to be produced by the loudspeaker, resulting in a loss of sound purity.